Soil pH Calculator
pH decides which nutrients your plants can actually absorb. Enter your bed area, current and target pH, and soil type to get the pounds of lime (raise pH) or sulfur (lower pH) â estimated from extension guidance.
How much amendment do you need?
Estimates from extension guidance â a lab soil test gives the exact rate for your soil. Most vegetables prefer pH 5.8â6.8.
Why pH matters more than fertilizer
You can pour on all the fertilizer you want â if pH is off, roots can't take it up. In strongly acidic soil, phosphorus, calcium and magnesium get locked away; in alkaline soil, iron, manganese and zinc disappear. That's why pale, stunted plants often need a pH fix, not more nitrogen. Most vegetables are happiest between pH 5.8 and 6.8; blueberries and azaleas are the famous exceptions, wanting ~4.5â5.5.
The amendment rates
Amounts are per 100 square feet to move pH by one full point (e.g. 5.5 â 6.5). Clay needs more because it buffers pH changes; sand needs less.
| Soil type | Lime to raise 1.0 pH | Sulfur to lower 1.0 pH |
|---|---|---|
| Sandy | 5 lb | 1 lb |
| Loam | 7 lb | 1.5 lb |
| Clay | 8 lb | 2 lb |
Ranges compiled from Cornell, UConn and UF/IFAS extension guidance. Your soil test lab's own lime requirement always wins over a table.
Worked examples
Raising pH: a 200 sq ft loam bed at pH 5.5, target 6.5 â that's 1.0 point à 7 lb à (200/100) = 14 lb of lime. But 7 lb per 100 sq ft exceeds the 5 lb single-application limit, so split it: 7 lb now, 7 lb next season.
Lowering pH: a 100 sq ft clay bed at pH 7.5, target 6.5 â 1.0 point à 2 lb à 1 = 2 lb of elemental sulfur, applied as two 1-lb doses a month apart (sulfur needs warm soil above ~50°F and 2â3 months to react).
Timing and technique
- Lime in fall. Ground limestone takes 3â6 months to fully react â spread in fall for next spring's garden, and retest before planting.
- Work it in. Till amendments into the top 6 inches rather than leaving them on the surface.
- Sulfur in small doses. ÂŊâ1 lb per 100 sq ft at a time, at least a month apart; bacteria do the work, so warmth and moisture matter.
- Don't guess â soil test. A $20â40 lab test (5â10 sub-samples from 4â6 inches deep, mixed) beats any calculator for the exact rate.
Frequently asked questions
How much lime raises soil pH by 1 point?
Per 100 sq ft: ~5 lb for sandy soil, 7 lb for loam, 8 lb for clay. Extension-guidance estimates â a soil test gives your exact rate.
How much sulfur lowers soil pH by 1 point?
Per 100 sq ft: ~1 lb for sandy soil, 1.5 lb for loam, 2 lb for clay. Apply in ÂŊâ1 lb increments a month apart.
How long does lime take to work?
3â6 months for ground limestone. Apply in fall for the spring garden; retest before planting.
Can I apply too much lime at once?
Yes â never more than 5 lb per 100 sq ft in one application. Split larger amounts across seasons; over-liming locks up micronutrients.
Do I really need a soil test first?
For the exact rate, yes: "don't guess, soil test." Most vegetables want pH 5.8â6.8; blueberries want ~4.5â5.5.